Embodiments of the invention relate to vehicular communications employing orthogonal frequency-division multiplexing (OFDM). OFDM is known and widely used in fixed WLAN environments. A common acquisition method for OFDM in WLAN applications is described in T. M. Schimdl and D. C. Cox, “Robust frequency and timing synchronization for OFDM,” IEEE Transactions on Communications, vol. 45, pp. 1613-1621, December 1997, (hereinafter the “SC method”). The acquisition enables detection of a frame boundary.
Typical WLAN environments addressed by the SC method include homes and offices, i.e. indoor, normally line-of-sight (LOS) environments. The SC method detects a frame boundary through use of short preamble (SP) delayed copy correlation (DCC) applied to OFDM data inputs received in a strongest (highest energy) path. The frame boundary detection is done before a long preamble (LP) period begins.
There is a growing interest in applying OFDM in vehicular communications, e.g. in vehicle-to-vehicle (V2V) communications. In contrast with indoor communications, OFDM signal acquisition in vehicular communications presents at least two major challenges: longer ranges and severe multipath effects in non-line-of-sight (NLOS) environments. These challenges are explained through examples described with reference to FIGS. 1-3.
FIG. 1 illustrates a common NLOS environment for V2V communication between two vehicles 100 and 104. A short (direct) path 106 passing through a building 102 has larger attenuation than a second (longer) path 108 which arrives at vehicle 104 later in time. FIG. 2 illustrates the received signal of the two paths. Path 106 has lower energy than path 108. The use of SP-DCC in the SC method would result in a maximal path energy value erroneously related to the strong (second) path. Consequently, a point 206 will be detected instead of a point 204 as a frame boundary. This will cause a Fast Fourier Transform (FFT) window 208 to be applied with an additional part of the next received symbol, S2(t+N), thereby causing Inter Symbol Interference (ISI) and hence Inter Carrier Interference (ICI). In other words, the SC method would fail in detecting the correct frame boundary, since it acquires the strongest (second) path and not the first (weaker) path. Modem performance will typically be degraded by several dBs when the wrong path is selected. Known variants of the SC method are based essentially on similar “strongest path” acquisition, and may fail in a similar way to detect a frame boundary correctly.
Another problem with the SC method lies in its reliance on SP. As implied by its name, SP is indeed short, only 0.8 μsec in a 20 MHz channel and 1.6 μsec in a 10 MHz channel. The power delay profile (PDP) of an outdoor channel may be close to, or even exceed, such values. In this case, reliable integration is impossible because the integration needs to be long enough to sum all channel path energies, but short enough to avoid confusion between repeated occurrences of SP replicas on the same path (which occur every SP length). FIG. 3 shows the PDP of channel HiperLan-E (defined in J. Medbo and P. Schramm, “Channel models for HIPERLAN/2,” ETSI/BRAN document no. 3ERI085B). |H|2 is the channel energy squared. It is seen that the path at 300 nsec is stronger than the direct (first) path at 0 nsec. The path at 150 nsec is also stronger than the path at Onsec (although weaker that that at 300 nsec). In outdoor communication channels, the discrepancy would be even greater.
There is therefore a need for, and it would be advantageous to have a method for OFDM frame boundary detection in a vehicular multipath channel which does not suffer from the disadvantages inherent in methods which have been developed for, and mainly applied in indoor environments.